Performance Evaluation of the uEXPLORER Total-Body PET/CT Scanner Based on NEMA NU 2-2018 with Additional Tests to Characterize PET Scanners with a Long Axial Field of View

Abstract

The world's first total-body PET scanner with an axial field of view (AFOV) of 194 cm is now in clinical and research use at our institution. The uEXPLORER PET/CT system is the first commercially available total-body PET scanner. Here we present a detailed physical characterization of this scanner based on National Electrical Manufacturers Association (NEMA) NU 2-2018 along with a new set of measurements devised to appropriately characterize the total-body AFOV. Methods: Sensitivity, count-rate performance, time-of-flight resolution, spatial resolution, and image quality were evaluated following the NEMA NU 2-2018 protocol. Additional measurements of sensitivity and count-rate capabilities more representative of total-body imaging were performed using extended-geometry phantoms based on the world-average human height (∼165 cm). Lastly, image quality throughout the long AFOV was assessed with the NEMA image quality (IQ) phantom imaged at 5 axial positions and over a range of expected total-body PET imaging conditions (low dose, delayed imaging, short scan duration). Results: Our performance evaluation demonstrated that the scanner provides a very high sensitivity of 174 kcps/MBq, a count-rate performance with a peak noise-equivalent count rate of approximately 2 Mcps for total-body imaging, and good spatial resolution capabilities for human imaging (≤3.0 mm in full width at half maximum near the center of the AFOV). Excellent IQ, excellent contrast recovery, and low noise properties were illustrated across the AFOV in both NEMA IQ phantom evaluations and human imaging examples. Conclusion: In addition to standard NEMA NU 2-2018 characterization, a new set of measurements based on extending NEMA NU 2-2018 phantoms and experiments was devised to characterize the physical performance of the first total-body PET system. The rationale for these extended measurements was evident from differences in sensitivity, count-rate-activity relationships, and noise-equivalent count-rate limits imposed by differences in dead time and randoms fraction between the NEMA NU 2 70-cm phantoms and the more representative total-body imaging phantoms. Overall, the uEXPLORER PET system provides ultra-high sensitivity that supports excellent spatial resolution and IQ throughout the field of view in both phantom and human imaging.

Keywords: EXPLORER; PET; performance evaluation; total-body imaging.

Graphical abstract

FIGURE 1.

Photograph of uEXPLORER total-body PET/CT scanner installed at EXPLORER Molecular Imaging Center in Sacramento, CA.

FIGURE 2.

Axial sensitivity profiles for 70-cm (NEMA NU 2-2018) (A) and 170-cm (B) line source phantoms. Sinogram slice thickness is 1.444 mm.

FIGURE 3.

(A and B) 70-cm-long NEMA NU 2 scatter phantom (A) and 175-cm-long scatter phantom (B) assembled from multiple NEMA NU 2 phantoms on uEXPLORER PET/CT patient bed. (C and D) Measured count-rates with 70-cm-long (C) and 175-cm-long (D) scatter phantom. Count-rate measures are plotted vs. left vertical axes; scatter fractions are plotted vs. right vertical axes. Activity concentrations for A and B were computed by dividing total activity in phantom at each time-point by phantom volume (22 L for 70-cm-long phantom and 55 L for 175-cm-long phantom).

FIGURE 4.

TOF resolution plotted vs. activity concentration using 70-cm-long NEMA NU 2 scatter phantom. TOF resolution of 505 ps at 5.3 kBq/mL was obtained.

FIGURE 5.

Reconstructed image slices of mini-Derenzo phantom imaged at axial center (A and B) and ⅟₁₆ AFOV (C and D) and with 2 orientations: transaxial (A and C) and sagittal (B and D). Image slice thickness is 1.172 mm.

FIGURE 6.

Contrast recovery (A) and background variability (B) measured with standard NEMA IQ phantom evaluation placed at center of AFOV and scanned for 30 min.

FIGURE 7.

Contrast recovery (A and B) and background variability (C and D) as function of scan duration (A and C) and activity (B and D). Percentage activity is relative to initial activity in phantom at starting time of scans. (E) Transaxial image slices of 30-min scan at several imaging time-points reconstructed using clinical protocol. All images are decay-corrected and use same color scale: 0–20 kBq/mL.

FIGURE 8.

Human imaging examples of performance of uEXPLORER total-body PET scanner. (A) Axial slice from ¹⁸F-fluciclovine PET image (right), with corresponding fused image (middle) and CT image (left), of 68-y-old patient with castration-resistant metastatic prostate cancer, demonstrating clear visualization of ¹⁸F-flucicovine accumulation within 2.5-mm-diameter pulmonary nodule. (B) Maximum-intensity projection of representative clinical oncology ¹⁸F-FDG PET scan reconstructed with 20-, 5-, and 2.5-min durations, of 59-y-old patient with lung cancer. Images show primary tumor in left lower lobe of lung (dashed circle), with multiple variable-sized (0.8–6 cm) hilar, mediastinal, and lower esophageal nodal metastases (arrows) and ∼1-cm ¹⁸F-FDG–avid left adrenal nodule (arrowhead), which is visualized for all scan durations.